pinctrl: microchip-sgpio: allow sgpio driver to be used as a module

As the commit message suggests, this simply adds the ability to select
SGPIO pinctrl as a module. This becomes more practical when the SGPIO
hardware exists on an external chip, controlled indirectly by I2C or SPI.
This commit enables that level of control.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220905162132.2943088-5-colin.foster@in-advantage.com